President-elect Donald Trump wrote a series of tweets early Sunday morning in which he threatened “retribution” in the form of a 35 percent tariff on U.S. companies that move abroad.

“The U.S. is going to substantialy (sic) reduce taxes and regulations on businesses, but any business that leaves our country for another country, fires its employees, builds a new factory or plant in the other country, and then thinks it will sell its product back into the U.S. without retribution or consequence, is WRONG!” Trump tweeted.

“There will be a tax on our soon to be strong border of 35% for these companies wanting to sell their product, cars, A.C. units etc., back across the border,” he added. “This tax will make leaving financially difficult, but these companies are able to move between all 50 states, with no tax or tariff being charged. Please be forewarned prior to making a very expensive mistake! THE UNITED STATES IS OPEN FOR BUSINESS.”

Experts have warned that putting up such tariffs would risk opening a trade war with countries Trump repeatedly bashed during his campaign, namely China and Mexico. In September, Trump said he intended to slap a similar 35 percent tariff Ford cars produced in Mexico and sold in the U.S.

Sen. Ben Sasse (R-Neb.), a prominent critic of Trump, said U.S. consumers would likely pay more as a result.

Trump announced earlier this week that he had reached a deal with Carrier that would keep about 800 of 1,400 jobs the company planned to move from the U.S. to Mexico. The arrangement seemed to be the product of not just sticks, but carrots as well ― $7 million in tax breaks over 10 years.